We're dealing with a similar summer camp + prep situation. For the structured test prep, a virtual 1:1 tutor who can flex around camp hours might be your best bet - we had good luck finding options on Wyzant when we needed scheduling flexibility.
For the vocabulary piece specifically, my husband built an app called Vocab Voyage (
https://vocab.voyage) that's been great for us. It turns vocab study into short game-based sessions - word challenges, speed rounds, crossword-style exercises — and uses spaced repetition so the words actually stick. The nice thing is your son can do it on his phone in 10-15 minute bursts between camp activities. No scheduling coordination needed.
Full transparency - my husband is the creator, so I'm biased! But it's completely free right now, no credit card or anything:
https://vocab.voyage
Hope you find a combo that works with the camp schedule - it's such a tricky juggle but starting over summer gives him a real advantage!